Power supply system and image forming apparatus having power supply system

ABSTRACT

A power supply system includes: a switching power supply generating a voltage from an alternating current power supply; capacitors connected to the alternating current power supply; a rectification circuit configured to rectify an alternating current voltage applied to the capacitors; a backflow regulation regulating backflow of current from the switching power supply; a signal generation circuit generating a detection signal corresponding to a zero cross point of the alternating current power supply; a power supply line connected to the switching power supply and the rectification circuit; an electricity storage unit connected to the power supply line; a voltage limit element configured to limit a voltage of the output line of the rectification circuit, and a control unit, power being fed to the control unit through the power supply line. The control unit is configured to detect a zero cross point based on the detection signal.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority ofJapanese Patent Application No. 2013-066393 filed on Mar. 27, 2013, thecontents of which are incorporated herein by reference in its entirety.

BACKGROUND

The present disclosure relates to a power supply system and an imageforming apparatus having the power supply system, and more particularly,to a technology of detecting a zero cross point of an alternatingcurrent voltage.

Conventionally, for example, JP-A-2010-239774 discloses a technology ofdetecting a zero cross point (zero cross timing) of an alternatingcurrent voltage. In the technical document, the zero cross point of thealternating current voltage is detected using a photo coupler.

SUMMARY

According to the method of detecting the zero cross point by using thephoto coupler, like the above technical document, the zero cross pointcan be favorably detected. However, the power consumption, which isconsumed by a photo diode of the photo coupler, is not negligibly low.For this reason, a technology of more saving power while maintainingdetection reliability of the zero cross point is seriously needed.

The aspect of the present disclosure provides a technology of moresaving power while maintaining detection reliability of a zero crosspoint.

one of aspects of the present disclosure provides the followingarrangements:

A power supply system comprising:

a switching power supply configured to rectify and smooth an alternatingcurrent voltage of an alternating current power supply and generate apredetermined direct current voltage;

a first capacitor including a first electrode and a second electrode,the first electrode being connected to one end of the alternatingcurrent power supply;

a second capacitor including a third electrode and a fourth electrode,the third electrode being connected to the other end of the alternatingcurrent power supply;

a rectification circuit configured to rectify an alternating currentvoltage applied to the first and second capacitors, the rectificationcircuit including an output line connected with an output line of theswitching power supply;

a backflow regulation element that is provided on the output line of therectification circuit and is configured to regulate backflow of currentfrom the switching power supply towards the rectification circuit;

a signal generation circuit connected to the output line of therectification circuit, the signal generation circuit configured togenerate a detection signal corresponding to a zero cross point of thealternating current power supply based on an output of the rectificationcircuit;

a power supply line positioned at a more downstream side than thebackflow regulation element and connected to the output line of theswitching power supply and the output line of the rectification circuit;

an electricity storage unit connected to the power supply line;

a voltage limit element configured to limit a voltage of the output lineof the rectification circuit to be lower than a voltage of the powersupply line, and

a control unit that is positioned at a more downstream side than theelectricity storage unit, power being fed to the control unit throughthe power supply line,

wherein the control unit is configured to detect a zero cross pointbased on the detection signal.

In the meantime, the power supply system disclosed in the specificationcan be applied to an image forming apparatus having an image formingunit that forms an image.

According to the present disclosure, it is possible to provide atechnology of more saving power while maintaining detection reliabilityof a zero cross point.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing an electrical configuration of amultifunction apparatus according to a first exemplified embodiment.

FIG. 2 is a circuit diagram of a power supply device (showing aswitching power supply-side).

FIG. 3 is a circuit diagram of the power supply device (showing a smallcapacity power supply circuit, a signal generation circuit, an electricdouble-layer capacitor, a control device and the like).

FIG. 4 shows a relation of an alternating current voltage of analternating current power supply and a pulse signal of the signalgeneration circuit.

FIG. 5 shows output timing of the pulse signal after power is input.

FIG. 6 shows a discharge path of a capacitor.

FIG. 7 is a circuit diagram (a small capacity power supply circuit, asignal generation circuit, an electric double-layer capacitor, a controldevice and the like) of the power supply device according to a secondembodiment.

FIG. 8 is a circuit diagram showing another exemplified embodiment ofthe power supply device.

DESCRIPTION OF EXEMPLARY EMBODIMENTS First Exemplified Embodiment

A first exemplified embodiment of the present disclosure will bedescribed with reference to FIGS. 1 to 6.

1. Multifunction Apparatus

FIG. 1 is a block diagram showing an electrical configuration of amultifunction apparatus (an example of the image forming apparatus) 1.The multifunction apparatus 1 includes a printing unit 2, a datacommunication unit 3 that is connected in communication with aninformation terminal apparatus 7, a FAX communication unit 4 that isconnected in communication with a FAX device 8, an image memory 5, adisplay unit 6 and a power supply system S. The power supply system S isconfigured by a power supply device 10 and a control device 100. Thepower supply device 10 is a power supply of the multifunction apparatus1 and feeds power to the printing unit 2, the data communication unit 3,the FAX communication unit 4, the image memory 5, the display unit 6 andthe control device 100. In the meantime, the printing unit 2 is anexample of the image forming unit.

The printing unit 2 includes a photosensitive drum 2 a, a charger 2 bthat executes a charging process of charging a surface of thephotosensitive drum 2 a, an exposure device 2 c that executes anexposure process of forming an electrostatic latent image on the surfaceof the photosensitive drum 2 a, a developing device 2 d that executes adeveloping process of attaching developer to the electrostatic latentimage formed on the surface of the photosensitive drum 2 a to thus forma developer image, a transfer device 2 e that executes a transferprocess of transferring the developer image to a recording medium, afixing device 2 f that executes a fixing process of fixing the developerimage transferred onto the recording medium, and the like.

The printing unit 2 executes the charging process, the exposure process,the developing process, the transfer process and the fixing process,thereby executing printing processing of printing print data or FAX dataon the recording medium. The data communication unit 3 performs datacommunication with the information terminal apparatus 7 such as a PC andreceives a printing instruction and print data from the informationterminal apparatus 7. The FAX communication unit 4 performs FAXcommunication with the FAX device 8 and receives FAX data from the FAXdevice 8. The image memory 5 temporarily stores therein the print dataor FAX data received from the information terminal apparatus 7 or FAXdevice 8.

When the data communication unit 3 receives a printing instruction fromthe information terminal apparatus 7 to thus receive print data and whenthe FAX communication unit 4 receives FAX data, the control device 100of the multifunction apparatus 1 enables the printing unit 2 to executethe printing processing consisting of the charging process, the exposureprocess, the developing process, the transfer process and the fixingprocess, thereby printing the print data or FAX data on the recordingmedium. In the meantime, while an operating voltage of the printing unit2 is 24V, operating voltages of the data communication unit 3, the FAXcommunication unit 4, the image memory 5, the display unit 6 and thecontrol device 100 are 3.3V.

2. Circuits of Power Supply System

A configuration of the power supply system S is described with referenceto FIGS. 2 and 3. The power supply device 10 includes a switching powersupply 20, a small capacity power supply circuit 40 which supply powerto the control device when the switching power supply is stopped(specifically, when the control IC 30 stops an on-off control of the FET25), an electric double-layer capacitor C6 serving as an electricitystorage unit, a signal generation circuit 60, a diode D8 for backflowregulation, and the like.

FIG. 2 is a circuit diagram showing a configuration of the switchingpower supply 20 of the power supply device 10 of the power supply systemS. The switching power supply 20 includes a rectification smoothingcircuit 21, a transformer 23, a FET (field effect transistor) 25, arectification smoothing circuit 27, a voltage detection circuit 29 and acontrol IC 30 that switching-controls the FET 25 and has a function ofconverting an alternating current (AC) voltage from an AC power supply15 to a predetermined direct current (DC) voltage and outputting thesame.

The rectification smoothing circuit 21 is a so-called capacitor inputtype and includes a bridge diode D1 that rectifies the AC voltage of theAC power supply 15 and a capacitor C1 that smoothes the rectifiedvoltage. An output-side of the rectification smoothing circuit 21 isprovided with the transformer 23, so that an input voltage Vin, which isobtained by rectifying and smoothing the AC voltage, is applied to aprimary coil N1 of the transformer 23.

The FET 25 is an N-channel MOSFET and has a drain D that is connected tothe primary coil N1 and a source S that is connected to a referencepotential of the primary-side. An on-off signal (PWM signal) is appliedfrom an output port OUT of the control IC 30 to a gate G, so that theFET 25 becomes on and off with a predetermined period. Thereby, theprimary-side of the transformer 23 oscillates, so that a voltage isinduced to a secondary coil N2 of the transformer 23.

The primary-side of the transformer 23 is provided with a voltagegeneration circuit 31. The voltage generation circuit 31 rectifies andsmoothes a voltage, which is induced to an auxiliary coil N3 provided tothe primary-side of the transformer 23, by a diode D2 and a capacitorC2. The voltage generation circuit 31 becomes a power supply (about 20V)of the control IC 30.

The rectification smoothing circuit 27 is provided at the secondary-sideof the transformer 23 and is configured by a diode D3 and a capacitorC3. The rectification smoothing circuit 27 rectifies and smoothes thevoltage induced to the secondary coil N2 of the transformer 23. Thereby,the switching power supply 20 outputs a voltage of DC 24V through anoutput line Lo1.

As shown in FIG. 2, the output line Lo1 is branched into three lines ata branch point J1, and the branched lines are provided with DC-DCconverters 35, 37, respectively. The DC-DC converter 35 drops the outputvoltage 24V of the switching power supply 20 to 5V and outputs the samefrom an output line Lo2. The DC-DC converter 37 drops the output voltage24V of the switching power supply 20 to 3.3V and outputs the same froman output line Lo3. Like this, the switching power supply 20 isconfigured to output the three voltages of 24V/5V/3.3V.

The voltage detection circuit 29 is provided between the rectificationsmoothing circuit 27 and the branch point J1 of the output line. Thevoltage detection circuit 29 is to detect a level of the output voltageVo1 (DC 24V) of the switching power supply 20 and is configured by apair of detection resistances R1, R2, a shunt regulator Re and a lightemitting diode LED1 that is connected in series with the shunt regularRe.

The detection resistances R1, R2 are provided between the output lineLo1 and a reference potential GND of the secondary-side and detect adivided voltage Vg that is obtained by dividing the output voltage Vo1by a resistance ratio. The shunt regulator Re enables current to flow,which corresponds to a level difference between a reference voltage inthe shunt regulator Re and the divided voltage Vg. Thereby, the currentflows through the light emitting diode LED 1 and the light emittingdiode LED 1 outputs a light signal having a light quantity correspondingto the level difference between the reference voltage and the dividedvoltage Vg.

The light emitting diode LED1 configures a photo coupler together with aphoto transistor PT1 connected to a feedback port FB of the control IC30. For this reason, the light signal of the light emitting diode LED 1is returned as an electric signal by the photo transistor PT1. Thereby,a signal (hereinafter, referred to as a feedback signal), whichindicates the level difference of the divided voltage Vg relative to thereference voltage of the shunt regulator Re, is input (feedback) to thefeedback port FB of the control IC 30.

As shown in FIG. 2, the control IC 30 includes a power supply port VCCthat is connected to the voltage generation circuit 31, a high voltageinput port VH that is connected to a power supply line through aresistance, the feedback port FB to which the feedback signal is input,an output port OUT that outputs an on-off signal (PWM signal) and an ENport to which a mode switching signal Sr (refer to FIG. 3) is input.

The control IC 30 includes a PWM comparator (not shown) and anoscillation circuit (not shown) that oscillates a triangular wave. Whenthe feedback signal is input to the feedback port FB, the control IC 30generates a PWM signal corresponding to the feedback signal and outputsthe same to the gate G of the FET 25 through the output port OUT.Thereby, the output voltage (DC 24V) of the switching power supply 20 iscontrolled to be a target voltage. Additionally, the control IC 30 has afunction of stopping and resuming the switching control (on-off control)of the FET 25, in response to the mode switching signal Sr that isoutput from the control device 100, which will be described later. Inthe below descriptions, a non-grounded side power supply line of a pairof power supply lines connected to the AC power supply 15 is referred toas a live LV-side and a grounded-side power supply line is referred toas a neutral NT-side.

The control device 100 has a main block B1 and a mode control block B2.The respective blocks B1, B2 may be configured by one or more CPUs, ahardware circuit such as an ASIC and the like or a combination of a CPUand a hardware circuit. In this exemplary embodiment, the respectiveblocks B1, B2 have memories 105, 85 embedded therein, respectively.

A power supply port P1 of the main block B1 is connected to the outputline Lo3 (refer to FIG. 2) of the DC-DC converter 37 and is fed with thepower from the switching power supply 20 through the DC-DC converter 37.The main block B1 controls the entire complex machine 1 including theprinting unit 2 and has a function of executing the printing processingof printing the FAX data and print data.

The mode control block B2 has a function of switching an operating modeof the switching power supply 20 between an output mode and a powersaving mode. Specifically, in the above exemplary embodiment, the panelswitch SW for instructing the switching of the operating mode isprovided for the display unit 6. The panel switch SW has one end that isconnected to a power supply line L3 and the other end that is connectedto an input port P4 provided to the mode control block B2. The other endis connected to the reference potential GND through a resistance. Forthis reason, when the user operates a panel switch SW2, a voltage of theinput port P4 is switched from a low level to a high level. In themeantime, the mode control block B2 is provided with a control port P3.The control port P3 is connected to a base of a transistor Q1 through aresistance to which a capacitor is connected in parallel. The transistorQ1 configure a switching circuit 90 for switching an operating mode ofthe switching power supply 20, together with a light emitting diodeLED2.

The transistor Q1 has an emitter that is connected to the referencepotential GND and a collector that is connected to a cathode of thelight emitting diode LED2. The light emitting diode LED2 has an anodethat is connected to the power supply line L3. The light emitting diodeLED2 configures the photo coupler together with a photo transistor PT2(refer to FIG. 2). A resistance is connected between a base and theemitter of the transistor Q1.

When the user operates the panel switch SW, the mode control block B2outputs the mode switching signal Sr from the control port P3, inresponse to the switching of the voltage of the input port P4. When themode switching signal Sr is output, the transistor Q1 becomes on, sothat the light emitting diode LED2 turns on and outputs a light signal.The light signal (the mode switching signal Sr) of the light emittingdiode LED2 is returned as an electric signal by the photo transistor PT2and is input to the EN port of the control IC 30. Then, the control IC30 stops and resumes the switching control (on-off control) of the FET25, in response to the input of the mode switching signal Sr, so thatthe operating mode of the switching power supply 20 is switched betweenthe output mode and the power saving mode.

In the meantime, the output mode is a mode where the control IC 30executes the switching control to thus switch the switching power supply20 to the output state. In the output mode, the complex machine 1 canimmediately execute the printing processing or the complex machine 1 isexecuting the printing processing, in response to the printinginstruction. Meanwhile, in the power supply system S, when the powersupply is input (a power supply plug is connected to the AC power supply15), the control IC 30 starts the switching control to thus shift theswitching power supply 20 to the output mode. The power saving mode is amode where the switching control of the control IC 30 is stopped to stopthe output of the switching power supply 20. In the power saving mode,the main block B1 of the control device 100 is disconnected as regardsthe power feeding from the switching power supply 20 and is thusstopped. In the meantime, the mode control block B2 is fed with thepower from the small capacity power supply circuit 40 and is thusoperated, which will be subsequently described.

The small capacity power supply circuit 40 is connected in parallel withthe switching power supply 20 with respect to the AC power supply 15. Inthe power saving mode where the switching power supply 20 is stopped,the small capacity power supply circuit 40 feeds the power to the modecontrol block B2 of the control device 100. Specifically, as shown inFIG. 3, the small capacity power supply circuit 40 has a first capacitorC4, a second capacitor C5 and a bridge-type rectification circuit 51.

The first capacitor C4 has a first electrode C4 p 1 and a secondelectrode C4 p 2. The first electrode C4 p 1 is connected to the liveLV-side of the AC power supply 15 and the second electrode C4 p 2 isconnected to the rectification circuit 51. The second capacitor C5 has afirst electrode C5 p 1 and a second electrode C5 p 2. The firstelectrode C5 p 1 is connected to the neutral NT-side of the AC powersupply 15 and the second electrode C5 p 2 is connected to therectification circuit 51. The first capacitor C4 and the secondcapacitor C5 have high impedances, which are alternating currentresistances, and perform a function of making a capacity of the smallcapacity power supply circuit 40 small, specifically, limiting outputcurrent of the rectification circuit 51 to about dozens to hundreds ofμA. In the meantime, the capacities of the first capacitor C4 and thesecond capacitor C5 are 3,300 pF (picofarad), for example.

The rectification circuit 51 is electrically connected between thesecond electrode C4 p 2 of the first capacitor C4 and the secondelectrode C5 p 2 of the second capacitor C5, thereby rectifying an ACvoltage Vac that is applied to both the capacitors C4, C5. Therectification circuit 51 is configured by a bridge circuit consisting offour diodes D4, D5, D6, D7, for example. Cathodes of the diode D4 andthe diode D5 are connected at a first connection point Nd1, an anode ofthe diode D4 is connected to the second electrode C4 p 2 of the firstcapacitor C4 and an anode of the diode D5 is connected to the secondelectrode C5 p 2 of the second capacitor C5.

Anodes of the diode D6 and the diode D7 are connected at a secondconnection point Nd2, a cathode of the diode D6 is connected to thesecond electrode C4 p 2 of the first capacitor C4 and a cathode of thediode D7 is connected to the second electrode C5 p 2 of the secondcapacitor C5.

An output line L2 of the rectification circuit 51 joins an output line(5V power output line Lo2) of the switching power supply 20 at a commonconnection point J2. A power supply line L3 having joined the two outputlines L2, Lo2 is connected to the power supply port P2 of the modecontrol block B2. Thereby, the power can be fed to the mode controlblock B2 from any of the switching power supply 20 and the smallcapacity power supply circuit 40. That is, in the output mode where theswitching power supply 20 is at the output state, the power is fed tothe mode control block B2 from the switching power supply 20 or smallcapacity power supply circuit 40. In the power saving mode where theoutput of the switching power supply 20 is stopped, the power is fedfrom the small capacity power supply circuit 40 to the mode controlblock B2.

An electric double-layer capacitor C6 serving as the electricity storageunit is provided at a downstream side close to the mode control blockB2, when seen from the common connection point J2. The electricdouble-layer capacitor C6 is provided between the power supply line L3and the reference potential GND. In the power saving mode where theoutput of the switching power supply 20 is stopped, the electricdouble-layer capacitor C6 functions as a power supply driving the lightemitting diode LED2 of the switching circuit 90, and functions as abackup power supply for feeding the power to an RTC (which is a circuitmeasuring current time and is used to detect FAX reception time) 80provided for the mode control block B2 upon the disconnection of the ACpower supply 15. Meanwhile, the RTC is an abbreviation of ‘real-timeclock’.

The signal generation circuit 60 is connected to the output line L2 ofthe rectification circuit 51. The signal generation circuit 60 is acircuit that generates a pulse signal Sp corresponding to an AC voltageVac of the AC power supply 15, and includes a resistance R3, atransistor Q2, a resistance R4, a zener diode ZD1 and a reverse voltageapplying circuit 70.

The transistor Q2 is an NPN transistor and has an emitter that isconnected to the reference potential GND and a collector connected tothe output line L2 through a resistance R4. The transistor Q2 has a basethat is connected to a connection point of the zener diode ZD1 and theresistance R3. The resistance R3 has one end that is connected to thebase of the transistor Q2 and the other end that is connected to theemitter of the transistor Q2. The zener diode ZD1 has an anode that isconnected to the base of the transistor Q2 and a cathode that isconnected to the output line L2.

A part of current Ip (current shown by chain line FIG. 3) that is outputfrom the rectification circuit 51 towards the signal generation circuit60 flows to the base of the transistor Q2 through the zener diode ZD1 orcapacitor C7.

As shown in FIG. 4, during a period (a period K1 in FIG. 4) for whichthe current Ip which flows from the rectification circuit 51 towards thesignal generation circuit 60 is higher than a reference value X, avoltage between the base and emitter of the transistor Q2 exceeds athreshold voltage and the current flowing through the base becomes apredetermined threshold value or larger, so that the transistor Q2becomes on. During a period (a period K2 in FIG. 4) for which thecurrent is lower than the reference value X, the current flowing throughthe base becomes smaller than the predetermined threshold value, so thatthe transistor Q2 becomes off. Since the rectification circuit 51rectifies the output of the AC power supply 15, the output of the signalgeneration circuit 60, i.e., the output (a potential of the collector)of the transistor Q2 becomes the pulse signal Sp corresponding to the ACvoltage Vac of the AC power supply 15. In this example, the pulse signalis a pulse signal having the same period as the AC voltage Vac andbecomes a signal having a phase that is offset with respect to the ACvoltage Vac. In the meantime, the transistor Q2 is an example of the‘switching device’ and the pulse signal Sp is an example of the‘detection signal’.

An output line of the signal generation circuit 60 (an output line drawnout from the collector of the transistor Q2) is connected to an inputport P5 of the mode control block B2 of the control device 100 and thepulse signal Sp output from the signal generation circuit 60 is input tothe mode control block B2. For this reason, the input pulse signal Sp isdata-processed in the main block B1 or mode control block B2 of thecontrol device 100, so that a zero cross point Zp of the AC voltage Vacof the AC power supply 15 can be detected.

Specifically, as shown in FIG. 4, a phase of the current Ip flowingthrough the resistance R3 progresses by 90° with respect to the ACvoltage Vac of the AC power supply 15. Therefore, the zero cross pointZP of the AC voltage Vac coincides with a center of the period (theperiod of a low level) K1 during which the pulse signal Sp is not outputor a center of the period (the period of a high level) K2 during whichthe pulse signal Sp is output. Hence, it is possible to detect the zerocross point Zp of the AC voltage Vac of the AC power supply 15 bydetecting the center of the period K1 during which the signal is notoutput and the center of the period K2 during which the signal isoutput.

For example, a zero cross point ZP1 shown in FIG. 4 can be obtained by afollowing equation 1.t4=t3+(K1/2)  (equation 1)

time t4 is occurrence time of the zero cross point ZP1 and time t3 isascending time of the pulse signal Sp.

A zero cross point ZP2 shown in FIG. 4 can be obtained by a followingequation 1.t6=t5+(K2/2)  (equation 2)

time t6 is occurrence time of the zero cross point ZP2 and time t5 isascending time of the pulse signal Sp.

In the meantime, the reason to detect the zero cross point ZP of the ACpower supply 15 by using the pulse signal Sp output from the signalgeneration circuit 60 is because the control device 100 controls theprinting unit 2 by using the detection of the zero cross point ZP.Specifically, the control device 100 performs control of an energizationamount (firing angle control or wave number control) on a heater 2 h(see FIG. 1) configuring the fixing device 2 f, based on the zero crosspoint ZP of the AC power supply 15. A signal Szc shown in FIG. 4 is azero cross signal that is generated in the control device 100 from adetection result of the zero cross point ZP. The zero cross signal Szcis used to detect timing of the zero cross point ZP when performing thecontrol of an energization amount of the heater 2 h. In addition tothis, it is possible to detect whether a power supply plug Y isconnected to the AC power supply 15, depending on whether or not thezero cross signal Szc.

The zener diode ZD1 has a function of limiting a voltage V2 of theoutput line L2 of the rectification circuit 51 to a voltage lower than avoltage V3 of the power supply line L3. That is, the voltage V3 of thepower supply line L3 is a voltage that is obtained by subtracting avoltage drop of the forward diode D9 from 5V, which is a line voltage ofthe output line Lo2 of the switching power supply 20. Since the voltagedrop of the forward diode D9 is slight and may be regarded as zero, thevoltage V3 of the power supply line L3 is about 5V.

In the meantime, the voltage V2 of the output line L2 of therectification circuit 51 is a voltage that is obtained by adding a zenervoltage (a breakdown voltage) of the zener diode ZD1 to the base voltageof the transistor Q2. In the power supply system S of this exemplaryembodiment, the zener voltage of the zener diode ZD1 is set to be 3.9V.The base voltage of the transistor Q2 is about 0.6V. For this reason,the voltage V2 of the output line L2 of the rectification circuit 51 islimited to 4.5V or lower. That is, a peak is cut at 4.5V. In this way,the voltage V2 of the output line L2 of the rectification circuit 51 isset to be a voltage lower than the voltage V3 of the power supply lineL3, so that it is possible to increase responsiveness of the signalgeneration circuit 60 upon the input of the power supply.

The reason is described as follows. When the power supply plug Y (referto FIG. 2) is connected to the AC power supply 15, the switching powersupply 20 is activated and is thus at the output state and the smallcapacity power supply circuit 40 is also at the output state. When thepower supply plug Y is connected at a state where the electricdouble-layer capacitor C6 is not charged, the charging current flowstowards the electric double-layer capacitor C6 through the power supplyline L3.

If the voltage V2 of the output line L2 of the rectification circuit 51is higher than the voltage V3 of the power supply line L3 (V2>V3), thecharging current flows not only from the switching power supply 20towards the electric double-layer capacitor C6 but also from therectification circuit 51 of the small capacity power supply circuit 40towards the electric double-layer capacitor C6.

For this reason, the current little flows through the resistance R3 ortransistor Q2 of the signal generation circuit 60 until the electricdouble-layer capacitor C6 is completely charged. Hence, since thetransistor Q2 is kept at the off state until the charging is completed,the pulse signal Sp is not output from the signal generation circuit 60and it is substantially impossible to detect the zero cross point ZP ofthe AC power supply 15.

Regarding the above problem, in the power supply system S of thisexemplary embodiment, the voltage V2 of the output line L2 of therectification circuit 51 is set to be lower than the voltage V3 of thepower supply line L3 (V2<V3). When the voltage V2 of the output line L2of the rectification circuit 51 is set to be lower than the voltage V3of the power supply line L3, the output current of the rectificationcircuit 51 does not flow towards the power supply line L3 but flowstowards the signal generation circuit 60 while the switching powersupply 20 is being activated.

Therefore, when the switching power supply 20 is activated as the powersupply plug Y is connected, the output current of the rectificationcircuit 51 mostly flows towards the signal generation circuit 60 along adashed-dotted line shown in FIG. 3, irrespective of whether the electricdouble-layer capacitor C6 is not charged. As a result, since the signalgeneration circuit 60 outputs the pulse signal Sp just after the powersupply plug Y is connected, it is possible to detect the zero crosspoint ZP of the AC power supply 15 just after the power supply plug Y isconnected (in an example of FIG. 5, about 0.1 s).

When the voltage V2 of the output line L2 of the rectification circuit51 is set to be lower than the voltage V3 of the power supply line L3,the current may flow back from the switching power supply 20 towardsrectification circuit 51 during the output of the switching power supply20, so that the transistor Q2 may be kept at the on state. Regardingthis, in the power supply system S of this exemplary embodiment, a diodeD8 for backflow regulation is provided on the output line L2 of therectification circuit 51. Therefore, it is possible to regulate thebackflow of the current from the switching power supply 20 towards therectification circuit 51 by the diode D8. In addition to the diode, atransistor or FET may be adopted as the backflow regulation element.However, since the diode is cheaper than the other elements, it has acost merit.

In the power supply system S of this exemplified embodiment, the signalgeneration circuit 60 is provided with the reverse voltage applyingcircuit 70. The reverse voltage applying circuit 70 includes a capacitorC7, a first discharge resistance R5 and a second discharge resistanceR3. The capacitor C7 is provided between the base of the transistor Q2and the output line L2 of the rectification circuit 51. The firstdischarge resistance R5 is connected between the output line L2 of therectification circuit 51 and the reference potential GND. The seconddischarge resistance R3 is connected between the emitter and base of thetransistor Q2.

The capacitor C7 is charged by the current Ip flowing to the signalgenerating circuit 60 through the path shown by the chain line of FIG.3. The capacitor C7 is discharged when the current Ip does not flow. Asshown with a dashed-dotted line in FIG. 6, the discharge current of thecapacitor C7 flows through a path of the first discharge resistance R5and the second discharge resistance R3. For this reason, as shown inFIG. 4, during the off-period of the transistor Q2, a voltage of thebase (the control terminal) becomes negative by an operation of thedischarge current of the capacitor C7. Hence, the on-off operations ofthe transistor Q2 become stable, without malfunction of the transistorQ2 caused due to noises. Therefore, since it is possible to detect asignal having a stable pulse, as the pulse signal Sp, it is possible toimprove the detection precision of the zero cross point ZP. In themeantime, a period hatched in FIG. 4 indicates a discharge cycle of thecapacitor C7 and the voltage of the base of the transistor Q2 becomesnegative during the period.

3. Effects

The output current (the rectified current) of the rectification circuit51 configuring the small capacity power supply circuit 40 issignificantly smaller by the operations of the first capacitor C4 andthe second capacitor C5, compared to the output current (the rectifiedcurrent) of the bridge diode D1 of the rectification smoothing circuit21 configuring the switching power supply 20. For this reason, it ispossible to detect the zero cross point ZP without using the photocoupler, by performing the zero cross detection method using the outputcurrent of the rectification circuit 51 configuring the small capacitypower supply circuit 40. Therefore, compared to the zero cross detectionmethod using the photo coupler, since it is not necessary to drive thephoto diode so as to detect the zero cross point, it is possible tosuppress the power consumption, thereby saving the power. Since theoutput of the rectification circuit 51 rectifies the AC output of the ACpower supply 15, it is possible to maintain the detection reliability ofthe zero cross point ZP by detecting the zero cross point ZP based onthe output of the rectification circuit 51.

In the power supply system S of this exemplary embodiment, the voltageV2 of the output line L2 of the rectification circuit 51 is set to belower than the voltage V3 of the power supply line L3 (V2<V3). For thisreason, the signal generation circuit 60 can output the pulse signal Spjust after the power supply is input, and it is possible to detect thezero cross point ZP of the AC power supply 15 just after the powersupply is input.

In particular, since an apparatus having the FAX function such as thecomplex machine 1 should record reception time of the FAX data, it ispreferable to provide the RTC circuit 80 measuring the current time.However, since the apparatus having the RTC circuit 80 embedded thereinshould operate the RTC circuit 80 even at a state where the power supplyis cut off, the capacity of the electricity storage unit (in thisexample, the electric double-layer capacitor C6) becomes larger. Forthis reason, it takes to charge the electricity storage unit after thepower supply is input and the signal generation circuit 60 does notsubstantially operate during the charging period, so that it takes tomake a state where it is possible to detect the zero cross point ZP ofthe AC power supply 15. When the power supply system S according to theexemplary embodiment is applied to the complex machine 1, it is possibleto considerably reduce the time after the power supply is input untilthe zero cross point ZP of the AC power supply 15 can be detected, whichis very effective, compared to a configuration where the power supplysystem of the related art is mounted.

Second Exemplary Embodiment

In the below, a second exemplary embodiment is described with referenceto FIG. 7.

In the second exemplary embodiment, the configuration of the circuit ofthe power supply system S of the first exemplary embodiment is partiallychanged. The same configurations as the first exemplary embodiment aredenoted with the same reference numerals and the descriptions thereofare omitted. The differences with the first exemplary embodiment aredescribed.

The circuit of the first exemplary embodiment has a problem in that whenthe output of the switching power supply 20 is stopped, the voltage ofthe electric double-layer capacitor C6 is gradually dropped from 5V andis finally lowered to 4.5V, which is the voltage of the output line L2of the rectification circuit 51.

Hence, in the power supply system S of the second exemplary embodiment,as shown in FIG. 7, a 24V output line Lo1 of the switching power supply20 is connected to the power supply line L3. For this reason, from astandpoint of a voltage balance, when the voltage V2 of the output lineL2 of the rectification circuit 51 is suppressed to at least 24V orlower, it is possible to maintain the relation of V2<V3. Thus, like thefirst exemplary embodiment, the signal generation circuit 60 can outputthe pulse signal Sp just after the power supply is input, and it ispossible to detect the zero cross point ZP of the AC power supply 15just after the power supply is input.

Therefore, in the power supply system S of the second exemplaryembodiment, the zener voltage of the zener diode ZD1 included in thesignal generation circuit 60 is set to be about 5.1V and the voltage V2of the output line L2 of the rectification circuit 51 is set to be avoltage (in this example, 5.7V) higher than 5V, which is a charge targetvoltage of the electric double-layer capacitor. Therefore, even when theswitching power supply 20 stops the output, it is possible to maintainthe voltage of the electric double-layer capacitor C6 at 5V that is acharge target voltage.

By the above configuration, compared to the power supply system S of thefirst exemplary embodiment, even when the switching power supply 20 isat a stop, it is possible to securely drive the light emitting diodeLED2 and to positively switch the operating mode of the switching powersupply 20. When the same charging amount is secured, a voltage drop issmaller, so that the power supply system S of the second exemplaryembodiment can make the capacity of the electric double-layer capacitorC6 smaller.

The power supply system S of the second exemplary embodiment has a zenerdiode ZD2 that is provided between the power supply line L3 and thereference potential GND. A zener voltage of the zener diode ZD2 is about5V and a voltage of a more downstream side of the power supply line L3than the zener diode ZD2 can be limited to about 5V. In this way, theinput voltage to the mode control block B2 can be suppressed to be awithstand voltage or lower.

A resistance R6 is provided on the power supply line L3 between theconnection point J2 of the output line L1 and a connection point J3 ofthe zener diode ZD2. The resistance R6 has a function of limiting thecurrent flowing from the switching power supply 20 towards the zenerdiode ZD2 through the output line Lo1. In this way, the resistance R6 isprovided to thus limit the current, so that the power consumption of theswitching power supply 20 can be suppressed.

In the power supply system S of the second exemplary embodiment, adedicated charging path L4 for the electric double-layer capacitor C6 isprovided. The charging path L4 connects the 5V power output line Lo2 ofthe switching power supply 20 to the electric double-layer capacitor C6through a forward diode D10. Since the charging path L4 has an impedancethat is set to be lower than the charging path passing through the powersupply line L3 (the high resistance R6 or resistance R7 is not provided,so that an impedance is lower), it is possible to charge the electricdouble-layer capacitor C6 in a short time.

Other Exemplary Embodiments

The invention is not limited to the above exemplary embodiments shown inthe drawings and following exemplary embodiments are also included inthe technical scope of the invention.

(1) In the first and second exemplary embodiments, the power supplysystem S is applied to the complex machine 1. However, the power supplysystem S can be applied to any electric device and the utility of thepower supply system S is not limited to the complex machine 1. Forexample, the power supply system S can be widely applied to homeappliances such as a television, a video recorder and the like. In thefirst and second exemplary embodiments, although the electrophotographicconfiguration has been exemplified as an example of the printing unit,an inkjet configuration is also possible.

(2) In the first and second exemplary embodiments, the signal generationcircuit 60 is provided with the reverse voltage applying circuit 70.However, the reverse voltage applying circuit 70 is not necessarilyrequired. That is, as shown in FIG. 8, the capacitor C7 and the firstdischarge resistance R5 may be omitted. In the first exemplaryembodiment, the resistance R4 of the signal generation circuit 60 isconnected to the anode of the diode D8. However, the resistance R4 maybe connected to the cathode of the diode D8.

(3) In the first and second exemplary embodiments, the diode D8 has beenexemplified as the backflow regulation element. However, a transistor orFET may be also used.

(4) In the first and second exemplary embodiments, the electricdouble-layer capacitor C6 has been exemplified as an example of theelectricity storage unit. However, any element may be used inasmuch asit can store the electricity, and a secondary battery and the like maybe also used. Also, an electrolytic capacitor may be used.

What is claimed:
 1. A power supply system comprising: a switching powersupply configured to rectify and smooth an alternating current voltageof an alternating current power supply and generate a predetermineddirect current voltage; a first capacitor including a first electrodeand a second electrode, the first electrode being connected to one endof the alternating current power supply; a second capacitor including athird electrode and a fourth electrode, the third electrode beingconnected to the other end of the alternating current power supply; arectification circuit configured to rectify an alternating currentvoltage applied to the first and second capacitors, the rectificationcircuit including an output line connected with an output line of theswitching power supply; a backflow regulation element that is providedon the output line of the rectification circuit and is configured toregulate backflow of current from the switching power supply towards therectification circuit; a signal generation circuit connected to theoutput line of the rectification circuit, the signal generation circuitconfigured to generate a detection signal corresponding to a zero crosspoint of the alternating current power supply based on an output of therectification circuit; a power supply line positioned at a moredownstream side than the backflow regulation element and connected tothe output line of the switching power supply and the output line of therectification circuit; an electricity storage unit connected to thepower supply line; a voltage limit element configured to limit a voltageof the output line of the rectification circuit to be lower than avoltage of the power supply line, and a control unit that is positionedat a more downstream side than the electricity storage unit, power beingfed to the control unit through the power supply line, wherein thecontrol unit is configured to detect a zero cross point based on thedetection signal.
 2. The power supply system according to claim 1,wherein the backflow regulation element is a diode.
 3. The power supplysystem according to claim 1, wherein the signal generation circuitincludes a switching device configured to be turned on and off based onthe output of the rectification circuit and generate the detectionsignal corresponding to the zero cross point of the alternating currentpower supply.
 4. The power supply system according to claim 3 furthercomprising a reverse voltage applying circuit configured to turn off theswitching device by applying, to a control terminal of the switchingdevice, a voltage having a reverse polarity to a voltage turning on theswitching device.
 5. The power supply system according to claim 4,wherein the switching device is a transistor having an emitter connectedto a reference potential-side and a collector connected to the outputline of the rectification circuit, and the reverse voltage applyingcircuit includes: a capacitor provided between a base of the transistorand the output line of the rectification circuit, a first dischargeresistance connected between the output line of the rectificationcircuit and the reference potential, and a second discharge resistanceconnected between the emitter and base of the transistor.
 6. The powersupply system according to claim 1, wherein the electricity storage unitincludes a dedicated charging path having impedance lower than acharging path passing through the power supply line.
 7. The power supplysystem according to claim 6, further comprising: a constant voltageelement configured to make a voltage of the power supply line be aconstant voltage that is a voltage lower than the output voltage of theswitching power supply; and a current limit element that is provided onthe power supply line and is configured to limit current flowing fromthe switching power supply towards the constant voltage element.
 8. Thepower supply system according to claim 1, further comprising a switchingcircuit configured to switch an output state where the switching powersupply generates the direct current voltage and a stop state where theswitching power supply stops an output, wherein the electricity storageunit stores electricity by output current of the rectification circuitor output current of the switching power supply, and wherein the storedelectricity becomes power for the switching circuit.
 9. An image formingapparatus comprising: a power supply system comprising: a switchingpower supply configured to rectify and smooth an alternating currentvoltage of an alternating current power supply and generate apredetermined direct current voltage; a first capacitor including afirst electrode and a second electrode, the first electrode beingconnected to one end of the alternating current power supply; a secondcapacitor including a third electrode and a fourth electrode, the thirdelectrode being connected to the other end of the alternating currentpower supply; a rectification circuit configured to rectify analternating current voltage applied to the first and second capacitors,the rectification circuit including an output line connected with anoutput line of the switching power supply; a backflow regulation elementthat is provided on the output line of the rectification circuit and isconfigured to regulate backflow of current from the switching powersupply towards the rectification circuit; a signal generation circuitconnected to the output line of the rectification circuit, the signalgeneration circuit configured to generate a detection signalcorresponding to a zero cross point of the alternating current powersupply based on an output of the rectification circuit; a power supplyline positioned at a more downstream side than the backflow regulationelement and connected to the output line of the switching power supplyand the output line of the rectification circuit; an electricity storageunit connected to the power supply line; a voltage limit elementconfigured to limit a voltage of the output line of the rectificationcircuit to be lower than a voltage of the power supply line, and acontrol unit that is positioned at a more downstream side than theelectricity storage unit, power being fed to the control unit throughthe power supply line; and an image forming unit that forms an image,wherein the control unit is configured to detect a zero cross pointbased on the detection signal, and control the image forming unit byusing the detection of the zero cross point.
 10. The image formingapparatus according to claim 9, wherein the image forming apparatusincludes a communication unit transmitting and receiving FAX data andhaving a FAX function, and the control unit includes an RTC circuitconfigured to measure current time.